RadAjaxManager wraps the ajaxfied controls in a div which is a "block" element - that's why the second listbox goes on a new line. You can fix it by setting UpdatePanelsRenderMode property of the ajax manager to "Inline".
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Yes, sorry, I meant to post again last night. I have discovered that. I'm a little surprised at it only seeming to affect this one page. I have a app with a couple of dozen ajaxified pages on that I'm currently updating; the old version was built using Q1 2009 and with the Q2 2010 controls none of those pages seem to have been affected, at least not that I've seen.